The study reveals that the subject of La Bella Principessa, painted by da Vinci before the Mona Lisa in the late 15th Century, also has an uncatchable smile, in which the shape of her mouth appears to change according to view point.

SHEFFIELD.- Mona Lisa might be famous for her enigmatic expression, but hers is not the only 'uncatchable smile' Leonardo da Vinci created, according to experts.

Academics at Sheffield Hallam University and the University of Sunderland have studied a lesser-known painting by the Renaissance master that shows evidence of the artistic skill that would later give his most famous portrait her mysterious allure.

The study reveals that the subject of La Bella Principessa, painted by da Vinci before the Mona Lisa in the late 15th Century, also has an uncatchable smile, in which the shape of her mouth appears to change according to view point.

When viewed directly, the slant of the mouth is distinctly downwards but as the eye moves elsewhere to examine other features, the mouth appears to take an upward turn, creating a smile that can only be seen indirectly, much like Mona Lisa's.

La Bella Principessa is thought to depict 13-year-old Bianca Sforza, the daughter of Ludovico Sforza ("Il Moro"), duke of Milan, who was to be married to a commander of the dukes Milanese forces.

But, she would be dead within months of the marriage, having suffered a possible ectopic pregnancy, adding poignancy to her expression in her portrait.

Alessandro Soranzo, from Sheffield Hallam's department of psychology, said: "The results from the experiments support the hypothesis that that there is a gaze-dependent illusory effect in the portrait of La Bella Principessa.

"Although it remains a question whether the illusion was intended, given Leonardos mastery of the technique and its subsequent use in the Mona Lisa, it is quite conceivable that the ambiguity of the effect was intentional, based on explicit artistic skill and used in line with Leonardos maxim that portraits should reflect some inner turmoil of the mind."

Michael Pickard, from the University of Sunderland and who co-authored the study, said: "With his knowledge of the turbulence surrounding the Court of Milan at that time, Leonardo would have been aware of inner tensions between the fresh innocence of a young girl on the threshold of womanhood and her impending marriage and courtly destiny.

"It is also not difficult to believe that Leonardo would have seen below the surface and wanted to capture the subtle essence of the girl, using a technique he would so famously master in the Mona Lisa."

The study La Bella Principessas Uncatchable Smile has been submitted to the Psychological Science Journal.